392 S. W. WILLISTON 
dermal armor completely and became fleet of movement, as is evi- 
denced by the structure of their limbs, limbs mimicking in form’and 
in structure so closely those of modern quick-running lizards as to be 
practically indistinguishable. We may be assured that some of them 
before the close of the Pennsylvanian were inhabitants of high-and- 
dry land regions where fleetness of movement, rather than obscurity, 
preserved them from their enemies, crawling reptiles in everything 
save some insignificant technical details of their palates. Specializa- 
tion of the microsaurs had reached the extraordinary extent of snake- 
like limbless forms. 
In addition to these two types of land animals we have two others 
which either persisted from unknown ancestors or made their advent: 
the temnospondylous type of amphibians from which the mammals 
eventually arose, and the stereospondylous type which terminated 
in the gigantic labyrinthodonts of the Upper Trias, the only group 
of the Pennsylvanian air-breathing vertebrates which we may say with 
certainty has left no modern descendants behind them. However, 
till near the close of the Pennsylvanian we have no knowledge of 
anything distinctive in the American land-vertebrate fauna. ‘There 
was nothing strikingly peculiar to either eastern or western continent, 
so far as our knowledge yet extends, and some of the forms, indeed, 
are almost if not quite identical generically. And the only possible 
explanation of this homogeneity of types is freedom of communica- 
tion and migration, the persistence and wide extent of like climatic 
and freshwater conditions that would.permit, for instance, the migra- 
tion of snake-like forms of small size from Ohio to Ireland and Bohe- 
mia without material modification in structure. 
However, either the divisional lines between the Pennsylvanian 
and the Permian have been placed too high, or else, it seems to me, 
evolution among the vertebrates was more rapid in America than 
elsewhere near the close of the period. As a continent I believe that 
the land of America was absolutely and continuously isolated, so far 
as the intermigrations of land forms was concerned, from some time 
before the close of the Pennsylvanian till well into Triassic times, as 
they reckon them in Europe. Of the Permian vertebrates by far the 
richest and most varied fauna known is that of America, and especially 
that of Texas and Oklahoma. Professor Case has recently presented 
